Processes and devices for controlling the braking force of at least one wheel of a vehicle are known in many variations. In these processes and devices, reference values are generally determined from measured values and estimated values. This essentially involves reference values which describe and/or influence the vehicle movement, such as wheel slip and/or yaw angle velocity. Proceeding from these reference values, braking moments are adjusted at the wheels of the vehicle. In the case of some of these conventional processes and devices, adjustment of the braking moments can take place individually by wheel.
In order to compensate varying dynamic performance (control response) which is caused, for example, by the temperature behavior of the components arranged in the device, or due to hydraulic and/or geometric tolerances, in some of these processes and devices a slight brake pressure is fed in, in the form of a time-limited filling pulse, before the pressure build-up which is necessary for adjusting the required braking moment occurs. As an alternative or supplement to this, in some devices and processes a correction of the valve control times is made.
German Patent Application No. 34 23 063 describes a traction control system for vehicles, in which a wheel is braked if a tendency for this wheel to slip occurs. In order to achieve rapid response of the brake when the tendency for the wheel to slip is detected, the brake is already applied at the very moment of the preliminary stages of the tendency to slip, by adjusting a slight brake pressure. In this connection, supply of the slight brake pressure is triggered as a function of the change in the throttle valve position, as a function of the vehicle speed, or as a function of slip thresholds which lie below the response thresholds for actual traction control. The slight brake pressure is supplied using a filling pulse with a constant time period. This means that when the need to supply the slight brake pressure exists, this pressure is supplied to the appropriate wheel brake cylinder for the predetermined period of time. Because of the low brake pressure supplied before the actual brake intervention, the reaction time which elapses from the time the tendency to slip has been determined until braking begins is reduced.
Since the filling pulse has a fixed, predetermined time period for all the wheels of a vehicle, tolerances which might be different for the individual wheels, for example, are not taken into consideration. This can result in differences in control response at the individual wheels, even though the slight brake pressure is supplied.
German Patent Application No. 196 04 126.0 describes a process and a device for controlling the brake system of a vehicle. In this conventional method and device, a build-up or reduction of the brake pressure in the brake cylinders is implemented by a pulse-like control of the inlet and outlet valves associated with the wheel brake cylinder. For this purpose, a pulse signal is used as the control signal, and this pulse signal can be changed in at least one parameter. Such possible parameters are the pulse length, the pulse pause period, the pulse height and/or the frequency of the pulse signal. In order to guarantee uniform pressure build-up dynamics in all operating situations, the parameter used in each instance is corrected as a function of a value which influences the pressure change dynamics. Appropriate values would be, for example, the temperature of the hydraulic oil or the ambient temperature and/or the supply voltage of the pump motor.
The task of the present invention consists in improving the compensation of varying control response within the device, i.e. within the process for controlling the braking force at at least one wheel of a vehicle.